In recent years, research and development of a light-emitting element (organic EL element) which uses an organic compound and utilizes electroluminescence (EL) have been actively promoted. In the basic structure of such a light-emitting element, an organic compound layer containing a light-emitting substance (an EL layer) is interposed between a pair of electrodes. By voltage application to this element, light emission from the light-emitting substance can be obtained.
Such a light-emitting element is a self-luminous element and has advantages over a liquid crystal display in having high pixel visibility and eliminating the need for backlights, for example; thus, such a light-emitting element is thought to be suitable for a flat panel display element. A display including such a light-emitting element is also highly advantageous in that it can be thin and lightweight. Besides, very high speed response is one of the features of such an element.
In such a light-emitting element, light-emitting layers can be successively formed two-dimensionally, so that planar light emission can be obtained. Thus, a large-area element can be easily formed. This feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, the light-emitting element also has great potential as a planar light source which can be applied to a lighting device and the like.
In the case of such an organic EL element, electrons from a cathode and holes from an anode are injected into an EL layer, so that current flows. By recombination of the injected electrons and holes, the organic compound having a light-emitting property is put in an excited state to provide light emission.
The excited state of an organic compound can be a singlet excited state or a triplet excited state, and light emission from the singlet excited state (S*) is referred to as fluorescence, and light emission from the triplet excited state (T*) is referred to as phosphorescence. The statistical generation ratio of the excited states in the light-emitting element is considered to be S*:T*=1:3.
In a compound that emits light from the singlet excited state (hereinafter, referred to as fluorescent substance), at room temperature, generally light emission from the triplet excited state (phosphorescence) is not observed while only light emission from the singlet excited state (fluorescence) is observed. Therefore, the internal quantum efficiency (the ratio of generated photons to injected carriers) of a light-emitting element using a fluorescent substance is assumed to have a theoretical limit of 25% based on the ratio of S* to T* which is 1:3.
In contrast, in a compound that emits light from the triplet excited state (hereinafter, referred to as phosphorescent compound), light emission from the triplet excited state (phosphorescence) is observed. Further, since intersystem crossing (i.e., transfer from a singlet excited state to a triplet excited state) easily occurs in a phosphorescent compound, the internal quantum efficiency can be increased to 100% in theory. That is, higher emission efficiency than in the case of using a fluorescent substance can be achieved. For these reasons, in order to achieve a highly efficient light-emitting element, a light-emitting element using a phosphorescent compound has been actively developed recently.
A white light-emitting element disclosed in Patent Document 1 includes a light-emitting region containing plural kinds of light-emitting dopants which emit phosphorescence.